1. Field
The present invention relates generally to communications systems, and more specifically, to systems and techniques for acquisition of a gated pilot signal.
2. Background
In wireless communications system, a user may access a network, or communicate with other users, through one or more base stations. Typically, each base station is configured to serve all users in a specific geographic region generally referred to as a cell. In some high traffic applications, the cell may be divided into sectors with a base station serving each sector. Each base station transmits a pilot signal which allows a user to synchronize with the base station and coherently demodulate the transmitted signal once the user is synchronized to the base station. The user generally establishes a communications channel with the base station having the strongest pilot signal.
The pilot signal is generally a known sequence embedded in the signal transmitted by the base station. By way of example, in code division multiple-access (CDMA) systems, the pilot signal can be characterized as an unmodulated signal spread with a known code. A user, or subscriber station, employs a locally generated replica of the spreading code to demodulate the pilot signal. The synchronization of the locally generated spreading code with the spread spectrum pilot signal is usually accomplished through an acquisition and tracking process. The acquisition process entails a search through an unknown region of time and frequency in order to bring the spread spectrum pilot signal into coarse alignment with the locally generated spreading code. Once the pilot signal has been acquired, a tracking loop can be used to optimize and maintain synchronization.
The ability of the subscriber station to acquire the pilot signal can be affected by numerous sources. By way of example, pilot signal acquisition can be severely hampered in the presence of residual frequency offsets in the baseband pilot signal due to local oscillator (LO) drift at the subscriber station or due to Doppler shift introduced by the relative motion between the subscriber station and base station. The difficulty in acquiring the pilot signal due to frequency offsets is a problem that is not unique to CDMA, but one that may be encountered in all communications systems.